1. Field of the Invention
The present invention relates to dry silver photothermographic imaging materials and to resistively developable photothermographic materials on polymeric substrates.
2. Prior Art
Photosensitive, heat-developable, dry silver sheet materials, as described for example in U.S. Pat. Nos. 3,457,075 and 3,839,049, contain a photosensitive silver halide catalyst-forming means in catalytic proximity with a heat sensitive combination of a light stable organic silver compound and a reducing agent therefor. When struck by light, the silver halide catalyst-forming means produces silver nuclei which serve to catalyze the reduction of the organic silver compound, e.g., silver behenate, by the reducing agent at elevated temperatures.
Color photothermographic imaging systems have been described in patent literature. U.S. Pat. No. 3,531,286 describes a system using paraphenylenediamine and photographic color couplers. U.S. Pat. No. 3,985,565 discloses the use of phenolic leuco dye reducing agents to reduce the silver and provide a color image. U.S. Pat. No. 4,460,681 discloses a multilayer color photothermographic system using a variety of leuco dyes separated by barrier layers.
It has been found to be desirable to provide a resistive layer in the photothermographic element which can be used as an integral heating means for the thermal development of the element. A voltage is applied across the resistive layer and the layer becomes heated, providing a uniform heat development of the exposed element. In order to utilize this type of resistive heating with transparent photothermographic media, it has been necessary to make the resistive layer strippable as shown in U.S. Pat. No. 4,409,316. Because the resist layer is most desirably a film with a high concentration of carbon black, the resist layer must be removed from the transparent substrate in order to allow viewing of the image.
Photothermographic media are also available with paper substrates. Resistive layers can be used on the backside of these photothermographic papers, but there are a number of sensitometric losses incurred. Paper substrates tend to cause more graininess and mottling than polymeric film substrates when coated with the same photothermographic emulsions, even when the paper substrate has a polymeric coating on its surface. Furthermore, paper is an insulator and the heating through that substrate tends to be less even than through a polymeric layer.